-nick

Just to fan the flames. Does a stock C2 on a dynojet make 215hp/209tq at the rear wheels? Or 199/187? At 12% loss, the first is close in hp, but overshoots torque. The second is low all over. You also can't expect the losses to be consistent across the revs. You can bet that frictional losses will increase with rpm / speed in gear. Are losses really a percentage, or a fixed value, or a combination? Don't trust absolute chassis dyno numbers!

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Steve Weiner-Rennsport Systems

Frictional losses can change with gearbox oil viscosity and tire pressures. Beyond that, its pretty fixed unless there are other issues such as internal transmission troubles.

Dynojets are far more consistent than other chassis dyno and that's why NASCAR, NASA and other major sanctioning bodies use them.

I see some issues on the power graphs that may indicate a little detonation. I'd suggest re-running the dyno and monitoring knock-sensor activity with a Hammer or PST-2.

MTR

Second car looks to be down on power. Could be a number of things: low compression on a cylinder, bad gas, throttle cable that isn't opening the throttle body 100% at WOT, etc. I didn't want to be disappointed so I tried to check and eliminate as many power robbing variables as I could before heading to the dyno. The owner of that car might want to run down a similar checklist.

MTR

Steve is that the waviness? Assuming detonation would cause the DME to pull timing, then when it stops detonating it would advance the timing and the power would increase causing the peaks and valleys?

Also, do you know if you can monitor injector duty cycle with a Hammer or PST-2?

porsche mania

No idea what dyno but here is my graph. I didn't really take it to find out the numbers more to find out the fuel/air ratio before the first track day after the rebuild.

These are the numbers at the wheels which I'm fairly pleased with. My butt dyno mirrors what the graph says in that it runs to the limiter fairly hard which has made me think I should raise it a little. The engine is freshly built, cup cams, balanced crank and rd flywheel, matched pistons, cases machined for windage/cylinders mooned also fitted lighter forged rods and ti valve hardwear with stiffer springs. Strangely enough the motronic is totally standard yet the afr readings were spot on, I've even had it checked by someone else who put the afr meter on the exhaust and tested it on the road. Would have loved 300 bhp but the wallet has taken a severe pounding already so ha e moved onto making the car liighter, made it to 1220kg now and it's made a serious difference!

-nick

I definitely won't pretend to know more than Steve on the subject, I'm just a natural skeptic. Dynojet charts do seem pretty consistent. Who knows what was going on with the 199hp car. Throttle not open all the way? Hot day in Philly and they left didn't change the temp correction? The latter is a reason to be suspicious of these methods of measurement. However, I would be pretty confident that Steve knows the real numbers that come out from his dyno.

This statement surprises me though "Frictional losses can change with gearbox oil viscosity and tire pressures. Beyond that, its pretty fixed unless there are other issues such as internal transmission troubles." Maybe it doesn't make much a real difference in hp? Spinning the wheels at 120mph is definitely going to make more heat in the drive line than spinning the wheels at 70mph. Heat is friction is drag is loss.

Here's another stock car, from Steve Wong's repository (we won't fault him for using 15% ha!). It's only +3hp and +1tq to the higher of the two fabspeed charts. Sounds like within the ballpark for car-to-car variation. Still, 12% tq correction would put the Wong and the higher fabspeed charts at +7ft-lbs over stock. Seems pretty unlikely.

If you want to kill some time with comparisons: http://911chips.com/dyno.html.

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-nick

Hey porsche mania, what are you using for exhaust? That's a big contribution.

porsche mania

I'm using standard headers with heat exchangers, cat bypass pipe straight into a twin outlet fabspeed rs back box

Steve Weiner-Rennsport Systems

Its one indication of detonation; reading the plugs is another, however that takes quite a bit of experience to glean accurate & useful information from. Since these ECU's can display knock-sensor activity using a factory scan tool, thats what I advise most people to do.

In an oblique way, yes. Its best to monitor AFR's and make any necessary changes from Lambda information.